In lithography techniques, for example, a resist film composed of a resist material is formed on a substrate, and the resist film is subjected to selective exposure of radial rays such as light or electron beam through a mask having a predetermined pattern, followed by development, thereby forming a resist pattern having a predetermined shape on the resist film.
A resist material in which the exposed portions become soluble in a developing solution is called a positive-type, and a resist material in which the exposed portions become insoluble in a developing solution is called a negative-type.
In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have lead to rapid progress in the field of pattern miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength (increasing the energy) of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are starting to be introduced in mass production. Furthermore, research is also being conducted into lithography techniques that use an exposure light source having a wavelength shorter (energy higher) than these excimer lasers, such as electron beam, extreme ultraviolet radiation (EUV), and X ray.
Resist materials for use with these types of exposure light sources require lithography properties such as a high resolution capable of reproducing patterns of minute dimensions, and a high level of sensitivity to these types of exposure light sources.
As a resist material that satisfies these conditions, a chemically amplified composition is used, which includes a base material component that exhibits a changed solubility in a developing solution under the action of acid and an acid-generator component that generates acid upon exposure.
For example, in the case where the developing solution is an alkali developing solution (alkali developing process), a chemically amplified positive resist which contains, as a base component (base resin), a resin which exhibits increased solubility in an alkali developing solution under action of acid, and an acid generator is typically used. If the resist film formed using the resist composition is selectively exposed during formation of a resist pattern, then within the exposed portions, acid is generated from the acid-generator component, and the action of this acid causes an increase in the solubility of the resin component in an alkali developing solution, making the exposed portions soluble in the alkali developing solution. In this manner, the unexposed portions remain to form a positive resist pattern. The base resin used exhibits increased polarity by the action of acid, thereby exhibiting increased solubility in an alkali developing solution, whereas the solubility in an organic solvent is decreased. When such a base resin is applied to a process using a developing solution containing an organic solvent (organic developing solution) (hereafter, this process is referred to as “solvent developing process” or “negative tone-developing process”) instead of an alkali developing process, the solubility of the exposed portions in an organic developing solution is decreased. Therefore, in a solvent developing process, the unexposed portions of the resist film are dissolved and removed by an organic solvent-type developing solution to thereby form a negative-tone resist pattern in which the exposed portions remain. For example, Patent Document 1 proposes a negative tone-developing process and a resist composition used for the process.
Currently, resins that contain structural units derived from (meth)acrylate esters within the main chain (acrylic resins) are now widely used as base resins for resist compositions that use ArF excimer laser lithography, as they exhibit excellent transparency in the vicinity of 193 nm (for example, see Patent Document 2).
In general, the base resin contains a plurality of structural units for improving lithography properties and the like. For example, in the case of a resin component which exhibits increased polarity by the action of acid, a base resin containing a plurality of structural units such as a structural unit having an acid decomposable group that is decomposed by the action of acid generated from the acid generator component, a structural unit having a polar group such as a hydroxyl group, a structural unit having a lactone structure, and the like is typically used. In particular, structural units having a polar group are widely used as they enhance the affinity for an alkali developing solution, and contributes to improvement in resolution.
Recently, a base resin having a structural unit containing an imide group has been proposed (see Patent Document 3). It is presumed that such as base resin contributes to improvement in resolution and mask reproducibility.